a) Field of the Invention
This invention relates to a real image mode finder optical system suitable for compact cameras such as cameras for photography with silver halide and video cameras.
b) Description of the Prior Art
In general, the finder optical system includes a virtual image mode such as an Albada type or an inverse Galilean type and a real image mode such as a Keplerian type. Most of the finder optical systems for compact cameras have adopted the virtual image mode in the past. This appears to result from the fact that the use of the finder optical system of the type, which dispenses with the need of a means for erecting an image, makes it possible to reduce the number of parts and diminish the length in a direction along an optical axis.
Recently, on the other hand, from the reasons that an indicating member like a field frame is easy of view, the effective diameter of a lens can be made small, and an image inverting optical member such as a Porro prism has come to be manufactured into integral forming of plastic, the use of real image mode finder optical systems has been increasing which, for instance, are set forth in Japanese Patent Preliminary Publication Nos. Sho 61-156018 and Sho 63-44616.
A typical example of such prior art is fundamentally constructed as shown in FIG. 1 (sectional view), in which reference numeral 1 represents an objective lens, 2 an image inverting optical member which is a Porro prism, and 3 an eyepiece.
FIGS. 2 and 3 are a developed view and aberration curve diagrams, respectively, of the optical system excluding the objective lens 1 from the conventional example, and its numerical data are shown below.
______________________________________ r.sub.1 = 25.0334 d.sub.1 = 36.0000 n.sub.1 = 1.49216 .nu..sub.1 = 57.50 r.sub.2 = -16.7986 d.sub.2 = 0.2000 r.sub.3 = 7.2998 (aspherical surface) d.sub.3 = 2.0000 n.sub.2 = 1.49216 .nu..sub.2 = 57.50 r.sub.4 = 8.1714 d.sub.4 = 15.0000 r.sub.5 = (pupil) ______________________________________ Aspherical coefficients Third surface E = -0.11367 .times. 10.sup.-3, F = -0.49046 .times. 10.sup.-7 G = -0.83477 .times. 10.sup.-7 ______________________________________
In the above conventional example, however, an image of an object formed through the objective lens 1 is assumed to be located adjacent to a first surface. Reference symbols r.sub.1, r.sub.2, . . . represent radii of curvature of individual lens surfaces, d.sub.1, d.sub.2, . . . thicknesses of individual lenses and spaces therebetween, n.sub.1, n.sub.2, . . . refractive indices of individual lenses, .nu..sub.1, .nu..sub.2, . . . Abbe's numbers of individual lenses, and E, F and G aspherical coefficients of fourth, sixth and eighth orders, respectively. The configuration of the aspherical surface in the conventional example is expressed by the following equation using the aspherical coefficients: ##EQU1## where X is the distance from the vertex of the aspherical surface in a direction parallel to the optical axis, S is the distance from the vertex of the aspherical surface in a direction perpendicular to the optical axis, and C is the curvature (=1/r) at the vertex of the aspherical surface.
In the preceding real image mode finder optical system of the conventional example, however, the position of the object image formed by the objective lens 1 is located in front of the image inverting optical member 2, behind which the eyepiece 3 is arranged, so that the length of the optical system in the direction along the optical axis is limited in reduction.
Thus, in order to solve such a problem, optical systems are proposed in which the imaging position provided by the objective lens 1 is located inside the image inverting optical member 2 with the intention of reducing the overall length, as is set forth in Japanese Patent Preliminary publication Nos. Sho 63-226616 and Hei 1-255825. Such prior art, however, has defects that since the field frame or the like is disposed at the imaging position, a component which can be originally configured or worked as a unit, for example, the Porro prism, must be divided into two elements, with the resultant increase of the number of parts, and the arrangement of the eyepiece 3 disposed behind the image inverting optical member 2 may cause the eyepiece 3 to protrude from the camera body, according to the position of the image inverting optical member.